The openings in aircraft skins are usually referred to as manholes, while the covers of these openings are commonly referred to as manhole covers or doors. Therefore, manholes and the covers or doors in aeronautical structures are a very widespread practice required for both assembly and subsequent maintenance and inspection tasks throughout the entire lifetime of the aircraft.
Aircraft access requirements may vary depending on the kind or type of access needed as well as on the frequency of its use.
Generally any structure in the aircraft comprising a hole or opening requires reinforcement to counteract the effect of the mentioned opening.
Access to fuel tanks is necessary in aeronautical structures. For the particular case of aircraft wings and stabilizers, this access is typically through manholes, and such manholes must meet the following requirements:                the size must be in accordance with accessibility requirements;        the door or cover must be sealed for the purpose of preventing fuel loss;        impact requirements must be met;        the assembly and disassembly of doors or covers must be as quick and as easy as possible;        the doors or covers must be changeable.        
There are different known solutions for this type of door or cover and manhole that are currently used in aircraft stabilizer and wing structures. These elements are generally arranged in the lower skins of stabilizer and wing structures.
Manholes typically used in horizontal stabilizers differ from the generally used concept for the same manhole doors in the metal skins on wings.
Therefore one of the known solutions used for metal aircraft structures typically used in the lower skins on aerodynamic surface wings consists of an opening with suitable dimensions in which two separate covers or doors are located, one inside the fuel tank and the second one on the aerodynamic surface, screwed together such that they secure the panel of skin between them. To maintain the aerodynamic surface, a cavity is machined on the outer surface of the skin to install the outer cover or door.
Both parts are screwed together to close the cover or door. The resulting pressure on the surrounding surface seals the door through the action of sealing profiles.
However the solution currently used for manholes for aircraft structures made of carbon fiber differs from the solution described above. Metal and carbon fiber have different properties. Carbon fiber furthermore has a much lower capacity for machining than metal does, and therefore the cavity machined on the metal skin described above is a very complex process for skins made from carbon fiber.
This leads to a different concept of manhole that does not require machining cavities in the skin. In addition this also prevents the skin from being perforated in several positions for installing the screws that will close the cover or door of the aircraft manhole.
This known type of cover or door comprises the following elements:                an inner cover screwed to the skin with countersunk head screws; a sealing profile between the cover and the spacer ring is generally necessary to prevent losses;        a spacer ring attached to the inner face of the skin to obtain a suitable flat surface for correct positioning of the cover;        an outer plate screwed to the inner cover giving the assembly a suitable aerodynamic surface;        pigtails for electric coupling.        
This type of known cover for aircraft structures made from a composite material present sealing and tightness problems and are not suitable when these conditions are extreme. Furthermore they are not viable when the stringers of the structure are arranged very close together, which is a very common requirement in current aircraft design. These known covers particularly present problems when they must be arranged on surfaces on which stringers are supported when these surfaces must have a smooth profile and thicknesses with no sudden changes.
The present invention aims to meet these demands.